This invention relates generally to threaded inserts, and more particularly to a threaded insert having keys which are positioned within 90° straight wall slots.
Threaded inserts of the type to which this invention is directed are ordinarily used to provide strong, permanent threads in soft or ductile parent materials, especially when a mating stud or bolt is to be frequently removed. Threaded inserts are also used to repair stripped, damaged or worn threads in a parent material. Soft or ductile materials which have received threaded inserts typically exhibit improved load-carrying capacity under static and dynamic loading conditions.
Referring to FIGS. 1a, 1b and 1c, a prior art threaded insert includes a bushing portion having a cylindrical shape with a central bore throughout the length of the threaded insert. The bushing portion also includes female threads throughout the length of the surface within the central bore and male threads throughout the length of the external surface of the bushing portion. The male threads on the external surface of the bushing portion are configured to mate with a threaded hole within a parent material, such as a plate or casting made of aluminum or magnesium. The female threads within the central bore are configured to mate with a male threaded fastener, such as a bolt or stud. The threaded insert is typically made from a material which is stronger than the parent material, such as steel.
To ensure that the prior art threaded insert does not rotate within the threaded hole in the parent material when a male threaded fastener is being installed or removed, the threaded insert includes at least one key. The at least one key is ordinarily positioned within a dovetail slot which extends longitudinally along the external surface of the threaded insert. The depth of the dovetail slot is typically greater than the depth of the male threads on the external surface of the threaded insert.
The dovetail slot is added to the threaded insert through a broaching operation. The broaching operation includes forcing an elongated, serrated cutting tool having sequentially larger teeth in the form of the dovetail slot along the external surface of the threaded insert. During manufacturing of the threaded fastener, the broaching step generally requires an additional machine set-up, which can lead to increased manufacturing costs. Also, broaching tools are often relatively expensive compared to other types of tools, thus adding further to the manufacturing costs of the threaded insert.
After the broaching operation, the dovetail slot is deburred to remove any protruding ragged edges that may have been raised on the male threads on the external surface of the threaded insert during the broaching operation. The deburring operation is often performed by hand, which can add to the time and cost required to fabricate the threaded insert.
The key includes a tang portion and a locking portion with the tang portion and the locking portion being positioned adjacent each other and aligned longitudinally. A cross section of the tang portion includes a substantially rectangular shape, while a cross section of the locking portion includes a substantially triangular shape. The tang portion is positioned within the dovetail slot through an interference fit. An inside surface on the tang portion is preferably in contact with the bottom surface of the dovetail slot and the outside facing edges of the tang portion are broached into the angled surfaces of the dovetail slot. The thickness of the tang portion is sufficiently low that with the tang portion being in contact with the bottom surface of the dovetail slot, the outer corners of the tang portion remain deeper than the inside diameter of the male threads on the external surface of the threaded insert. This prevents the tang portion from interfering with the threads within the hole in the parent material during installation of the threaded insert. Prior to installation of the threaded insert into the parent material, the locking portion is positioned beyond the end of the bushing portion of the threaded insert. The thickness of the locking portion is sufficient that the outermost portion of the locking portion is positioned substantially flush with the outside diameter of the male threads on the external surface of the threaded insert.
During installation of the key into the dovetail slot, the inside surface of the tang portion of the key is substantially aligned with the bottom surface of the dovetail slot at an end of the threaded insert. A longitudinal force is applied to the key so that the tang portion of the key is pushed into the dovetail slot with the outside edges of the tang portion being broached into the angled surfaces of the dovetail slot. With no positive forces pushing the tang portion against the bottom surface of the dovetail slot during installation of the key into the dovetail slot, the inside surface of the tang portion may inadvertently be installed offset from the bottom surface of the dovetail slot. Therefore, a key resetting operation is performed to ensure that the tang portion is not positioned within the male threads on the external surface of the threaded insert. The key resetting operation includes applying a force to the tang portion of the key in a direction substantially perpendicular to the bottom surface of the dovetail slot. For those keys which are located with the tang portion offset from the bottom surface of the dovetail slot, the key resetting operation may cause the key to be pushed out of the newly broached portion of the angled surfaces of the slot and to become loose, which may be a sufficient reason to scrap the threaded insert. Also, the key resetting operation is a process which increases the manufacturing cost of the threaded insert.
At installation, the threaded insert may be screwed into the threaded hole in the parent material until the locking portion of the at least one key on the threaded insert contacts a surface of the parent material. The at least one key is then driven into the parent material so that the locking portion of the key broaches through the threads of the threaded hole in the parent material and remains in place within the threads in the parent material. Having the key in place within the threads in the parent material locks the threaded insert in place by preventing further rotation of the threaded insert within the threaded hole in the parent material. While driving the at least one key into the parent material, the tang portion of the key is simultaneously broached further into the angled surfaces of the dovetail slot, thereby adding to the difficulty of driving the key into the parent material.
In light of the foregoing description, those skilled in the art have recognized the need for a threaded insert having at least one locking key but having less tendency to broach into the slot within the threaded insert. The need for a threaded insert which can be manufactured more economically and with improved retention and alignment of the keys has also been recognized. Further, the need for a threaded insert which is configured for improved key driving has been recognized. The present invention fulfills these needs and others.